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Originally Posted by Mr Mephisto
"Indeed," I continued, "there is no reason to believe what you interpret as red, what you see as red, is what I see as red."
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Ignoring part of this problem, people's eye pigments are actually different from each other.
Colours actually look different to different people, and I mean this in a formal, testable way.
You can have two lights which one person sees as identical, but another sees as two different colours.
This is most clear when you think about colour blindness -- they'll see fewer colours, and more colours will be identical for them than for you. But, it happens in every person.
As such:
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Originally Posted by asaris
But that's not quite correct. It's not simply that the book appears blue to me, it's the fact that the book appears blue to almost everyone who can see it.
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There exists a book that looks blue to you, but doesn't look blue to someone else. Or, at the very least, looks like a quite different shade of blue to someone else.
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Originally Posted by Master_Shake
The colours are just wavelengths of light intepreted by our brains. I've always been amused that although we claim certain things have the property of a colour those things actually have every other colour property except that one we claim it has.
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We don't actually see wavelengths of light. We see how three (or so) distinct pigments in our eyes react to all of the light that enters our eyes and reaches a point on our eye.
Each pigment has a response curve. If you graph the light emitted, then multiply against the response curve of the pigment, then integrate the result, you get a value that describes how much the pigment will be excited by the incoming light.
The ratio and magnatude of the excitation of the 3 pigments, together with some other (far far more complicated) effects, is then used to determine the colour we percieve.
The 'more complicated' effects include effects that are used by optical illusions.
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Originally Posted by Master_Shake
For example, when we claim a chair is blue, we are mistaken. It is actually every other colour except blue. Every other visible light wavelength is absorbed by the chair, it only reflects the wavelength we associate with blue.
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Now you are being silly. We use the term 'blue' to mean 'when looked at, we see blue'.
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Originally Posted by Mr Mephisto
Do colours exist? Do you believe there is such a thing as red? As blue? As yellow? Does the object have an inherent characteristic, independent of its observation by a conscious observer, that can be described as a colour?
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It exists as much as weight exists. That object in that particular space-time region does emit photons.
Now, the standard description of the photons it lets off (redish, blue, yellow, etc) aren't all that percice. It is like saying "it is a heavy rock" or "it is a rock that is lighter than a car, but heavier than a person". A very rough measurement.